Blender Source File Path
Just to illustrate cegaton's comment.  

Filepaths in Blender are saved as relative by default.

You can still change them to absolute/relative though.
File > External Data > Make all paths relative/absolute.    



You can also change this default behavior in preferences:
Edit > Preferences > Save &amp; Load



On top of that, you can set default relative/absolute paths for some types of data (scripts, textures, etc...)
Just go to Edit > Preferences > File Paths